The present invention relates to a method and apparatus for discriminating operativeness/inoperativeness of an air-fuel ratio sensor which is provided in an exhaust passage of an internal combustion engine to detect an air-fuel ratio of mixture supplied to the internal combustion engine.
A feedback control system for an internal combustion engine which feedback-controls an air-fuel ratio of mixture to be supplied to the engine in response to the exhaust from the internal combustion engine has been employed to improve operating conditions of the internal combustion engine. The control system has an oxygen concentration sensor provided in the exhaust passage of the internal combustion engine as an air-fuel ratio sensor to detect the air-fuel ratio of mixture supplied to the engine and feedback controls quantity of fuel to be supplied to the internal combustion engine in response to the output signal of the oxygen concentration sensor. In other words, the system performs a feedback control to maintain the air-fuel ratio of mixture to be supplied to the combustion engine at a predetermined ratio by increasing and decreasing the quantity of fuel when the air-fuel ratio is above (lean) and below (rich) the predetermined ratio, respectively.
The control system, however, has not been satisfactory. When the oxygen concentration sensor is inoperative because of failure or malfunction thereof, but the air-fuel ratio of mixture to the internal combustion engine is still controlled in response to the output signal thereof, the air-fuel ratio of mixture is controlled to an excessively rich or lean side based on this erroneous output signal, thus deteriorating operating characteristics of the internal combustion engine. In addition, since the oxygen concentration sensor is inoperative or not activated sufficiently unless maintained above a high temperature, accurate air-fuel ratio feedback control cannot be performed without detecting operativeness/inoperativeness of the sensor.
There have been suggestions to discriminate operativeness/inoperativeness of the oxygen concentration sensor, as disclosed in U.S. Pat. No. 3,916,848, in which an output signal of the oxygen concentration sensor is compared with a predetermined signal level and the oxygen concentration sensor is discriminated to be inoperative when the oxygen concentration sensor does not change the output signal across the predetermined signal level within a predetermined interval of time.
This suggested operativeness/inoperativeness discrimination system, however, is not satisfactory. The conditions under which an air-fuel ratio detecting system, including the oxygen concentration sensor, fails to operate properly cannot accurately be predicted. Even if the oxygen concentration sensor changes the output signal across the predetermined signal level within the predetermined interval of time, the oxygen concentration sensor is not sufficiently operative for detecting the air-fuel ratio, when the sensor malfunctions so that the sensor output signal changes only slightly across the predetermined signal level.